Problem: In history class, the girl to boy ratio is $7$ to $3$. If there are $21$ girls, how many boys are there?
A ratio of $7$ girls to $3$ boys means that there is a group of $7$ girls for every group of $3$ boys. If there are $21$ girls, then there are $3$ groups of $7$ girls each. Because we know the ratio is $7:3$ , there must be $3$ groups of $3$ boys each. There is a total of $9$ boys in history class.